Conventionally, a transmission device such as a radio base station device in a mobile communication system amplifies a signal as a transmission target (hereinafter, “transmission signal”) by an amplifier. The amplifier is often caused to operate generally near a nonlinear characteristic portion in terms of improvement of amplification efficiency, and, in this case, there is a problem that distortion caused by nonlinear characteristic (hereinafter, “nonlinear distortion”) occurs in many cases. Meanwhile, if the amplifier is caused to operate in a linear characteristic portion, there is another problem that the amplification efficiency decreases although less nonlinear distortion occurs.
Therefore, in order to deal with the problems, there are proposed various technologies for operating the amplifier with high efficiency in the nonlinear characteristic portion and suppressing occurrence of the nonlinear distortion. For example, there is known a technology for receiving a transmission signal as an input signal and changing a voltage to be applied to the amplifier appropriately by following an envelope signal of the input signal (see Japanese Laid-open Patent Publication No. 2007-180795).
Here, a conventional transmission device that adopts the conventional technology will be explained below. FIG. 15 is a diagram of a configuration of the conventional transmission device. As represented in FIG. 15, a conventional transmission device 1 includes an amplifier 2, an envelope detector 3, and a voltage controller 4.
The amplifier 2 amplifies a transmission signal according to a voltage to be applied and outputs the amplified transmission signal to a transmission antenna 5. The envelope detector 3 detects an envelope signal of the transmission signal and inputs the detected envelope signal to the voltage controller 4.
The voltage controller 4 changes the voltage applied to the amplifier 2 according to the input envelope signal. For example, the voltage controller 4 generates a pulse signal with a pulse width according to the input envelope signal, and performs switching of the voltage applied to the amplifier 2 according to the pulse signal by using a predetermined switching device.
As explained above, the conventional transmission device 1 appropriately changes the voltage applied to the amplifier 2 according to the input signal, to thereby cause the amplifier 2 to operate in the nonlinear characteristic portion with high power efficiency.
However, the conventional transmission device has a problem that distortion different from nonlinear distortion may occur in the amplifier 2 if a communication system for handling a broadband transmission signal such as OFDM (Orthogonal Frequency Division Multiplexing) is used.
For example, in the OFDM communication system, because the envelope signal of the transmission signal changes at a high rate, switching by the voltage controller 4 of the transmission device may be delayed. Therefore, there occurs a phase difference between the voltage applied to the amplifier 2 by the voltage controller 4 and the transmission signal input to the amplifier 2. Consequently, distortion of the transmission signal occurs in the amplifier 2. It should be noted that in the present specification, the distortion occurring in the amplifier 2 caused by the phase difference between the voltage applied to the amplifier 2 by the voltage controller 4 and the transmission signal input to the amplifier 2 is called “voltage-control distortion”.
FIG. 16 is a diagram of schematic input/output gain characteristics of the amplifier 2. In FIG. 16, a horizontal-axis indicates a power level of an input signal and a vertical-axis indicates an input/output gain. If there occurs a phase difference between the voltage applied to the amplifier 2 by the voltage controller 4 and the transmission signal input to the amplifier 2, as represented in FIG. 16, an input/output gain fluctuates according to magnitude of the voltage V applied to the amplifier 2 even if the input signal to the amplifier 2 is at the same power level Pa. As explained above, the conventional transmission device has some problem that when there occurs the phase difference between the voltage applied to the amplifier 2 by the voltage controller 4 and the transmission signal input to the amplifier 2, the voltage-control distortion occurs in the amplifier 2.